The molecular basis of movement in human platelets and a soil ameba are being studied by isolating and characterizing their contractile proteins and studying the interactions of the proteins in vitro. Both actin and myosin have been purified. The actins are very similar to muscle actin and the extent of this similarity will be determined by sequencing the Acanthamoeba actin. Both platelet and Acanthamoeba myosin have ATPase activity and bind to myosin, but in contrast to muscle myosin neither of these cytoplasmic myosins has its Mg-ATPase activated significantly by pure actin. Actin activation of Acanthamoeba myosin is achieved in the presence of a new "cofactor" protein. Platelet myosin forms small bipolar aggregates consisting of about 28 molecules in physiological salt solutions. Anti-bodies to platelet myosin do not cross react with skeletal or smooth muscle myosin. The antibodies to myosin are being used to determine the intracellular localization of myosin in platelets and non-muscle cells.